Methods and apparatus for treatment of respiratory disorders

ABSTRACT

Disclosed are methods, apparatus and systems for treating a respiratory disorder in a patient. The apparatus comprises a pressure generator configured to generate a flow of air so as to provide ventilatory support to the patient; a transducer configured to generate a flow signal representing a property of the flow of air; and a controller configured to analyse the flow signal to estimate the inspiratory volume and the expiratory volume of a breath of the patient and servo-control the degree of ventilatory support to adjust an estimated tidal volume toward a target tidal volume. A gain of the servo-control is dependent on a difference between the estimated inspiratory volume and the estimated expiratory volume. The method comprises operating an apparatus or system in a similar manner.

1 CROSS-REFERENCE TO RELATED APPLICATIONS

This application claims the benefit of Australian ProvisionalApplication Nos. 2017901773, filed May 12, 2017, the entire contents ofwhich is incorporated herein by reference.

2 BACKGROUND OF THE TECHNOLOGY 2.1 Field of the Technology

The present technology relates to one or more of the screening,diagnosis, monitoring, treatment, prevention and amelioration ofrespiratory-related disorders. The present technology also relates tomedical devices or apparatus, and their operation and use.

2.2 Description of the Related Art 2.2.1 Human Respiratory System andits Disorders

The respiratory system of the body facilitates gas exchange. The noseand mouth form the entrance to the airways of a patient.

The airways include a series of branching tubes, which become narrower,shorter and more numerous as they penetrate deeper into the lung. Theprime function of the lung is gas exchange, allowing oxygen to move fromthe inhaled air into the venous blood and carbon dioxide to move in theopposite direction. The trachea divides into right and left mainbronchi, which further divide eventually into terminal bronchioles. Thebronchi make up the conducting airways, and do not take part in gasexchange. Further divisions of the airways lead to the respiratorybronchioles, and eventually to the alveoli. The alveolated region of thelung is where the gas exchange takes place, and is referred to as therespiratory zone. See “Respiratory Physiology”, by John B. West,Lippincott Williams & Wilkins, 9th edition published 2012.

A range of respiratory disorders exist. Certain disorders may becharacterised by particular events, e.g. apneas, hypopneas, andhyperpneas.

Examples of respiratory disorders include Obstructive Sleep Apnea (OSA),Cheyne-Stokes Respiration (CSR), respiratory insufficiency, ObesityHyperventilation Syndrome (OHS), Chronic Obstructive Pulmonary Disease(COPD), Neuromuscular Disease (NMD) and Chest wall disorders.

Obstructive Sleep Apnea (OSA), a form of Sleep Disordered Breathing(SDB), is characterised by events including occlusion or obstruction ofthe upper air passage during sleep. It results from a combination of anabnormally small upper airway and the normal loss of muscle tone in theregion of the tongue, soft palate and posterior oropharyngeal wallduring sleep. The condition causes the affected patient to stopbreathing for periods typically of 30 to 120 seconds in duration,sometimes 200 to 300 times per night. It often causes excessive daytimesomnolence, and it may cause cardiovascular disease and brain damage.The syndrome is a common disorder, particularly in middle agedoverweight males, although a person affected may have no awareness ofthe problem. See U.S. Pat. No. 4,944,310 (Sullivan).

Cheyne-Stokes Respiration (CSR) is another form of sleep disorderedbreathing. CSR is a disorder of a patient's respiratory controller inwhich there are rhythmic alternating periods of waxing and waningventilation known as CSR cycles. CSR is characterised by repetitivede-oxygenation and re-oxygenation of the arterial blood. It is possiblethat CSR is harmful because of the repetitive hypoxia. In some patientsCSR is associated with repetitive arousal from sleep, which causessevere sleep disruption, increased sympathetic activity, and increasedafterload. See U.S. Pat. No. 6,532,959 (Berthon-Jones).

Respiratory failure is an umbrella term for respiratory disorders inwhich the lungs are unable to inspire sufficient oxygen or exhalesufficient CO₂ to meet the patient's needs. Respiratory failure mayencompass some or all of the following disorders.

A patient with respiratory insufficiency (a form of respiratory failure)may experience abnormal shortness of breath on exercise.

Obesity Hyperventilation Syndrome (OHS) is defined as the combination ofsevere obesity and awake chronic hypercapnia, in the absence of otherknown causes for hypoventilation. Symptoms include dyspnea, morningheadache and excessive daytime sleepiness.

Chronic Obstructive Pulmonary Disease (COPD) encompasses any of a groupof lower airway diseases that have certain characteristics in common.These include increased resistance to air movement, extended expiratoryphase of respiration, and loss of the normal elasticity of the lung.Examples of COPD are emphysema and chronic bronchitis. COPD is caused bychronic tobacco smoking (primary risk factor), occupational exposures,air pollution and genetic factors. Symptoms include: dyspnea onexertion, chronic cough and sputum production.

Neuromuscular Disease (NMD) is a broad term that encompasses manydiseases and ailments that impair the functioning of the muscles eitherdirectly via intrinsic muscle pathology, or indirectly via nervepathology. Some NMD patients are characterised by progressive muscularimpairment leading to loss of ambulation, being wheelchair-bound,swallowing difficulties, respiratory muscle weakness and, eventually,death from respiratory failure. Neuromuscular disorders can be dividedinto rapidly progressive and slowly progressive: (i) Rapidly progressivedisorders: Characterised by muscle impairment that worsens over monthsand results in death within a few years (e.g. Amyotrophic lateralsclerosis (ALS) and Duchenne muscular dystrophy (DMD) in teenagers);(ii) Variable or slowly progressive disorders: Characterised by muscleimpairment that worsens over years and only mildly reduces lifeexpectancy (e.g. Limb girdle, Facioscapulohumeral and Myotonic musculardystrophy). Symptoms of respiratory failure in NMD include: increasinggeneralised weakness, dysphagia, dyspnea on exertion and at rest,fatigue, sleepiness, morning headache, and difficulties withconcentration and mood changes.

Chest wall disorders are a group of thoracic deformities that result ininefficient coupling between the respiratory muscles and the thoraciccage. The disorders are usually characterised by a restrictive defectand share the potential of long term hypercapnic respiratory failure.Scoliosis and/or kyphoscoliosis may cause severe respiratory failure.Symptoms of respiratory failure include: dyspnea on exertion, peripheraloedema, orthopnea, repeated chest infections, morning headaches,fatigue, poor sleep quality and loss of appetite.

A range of therapies have been used to treat or ameliorate suchconditions. Furthermore, otherwise healthy individuals may takeadvantage of such therapies to prevent respiratory disorders fromarising. However, these have a number of shortcomings.

2.2.2 Therapy

Various therapies, such as Continuous Positive Airway Pressure (CPAP)therapy, Non-invasive ventilation (NIV) and Invasive ventilation (IV)have been used to treat one or more of the above respiratory disorders.

Continuous Positive Airway Pressure (CPAP) therapy has been used totreat Obstructive Sleep Apnea (OSA). The mechanism of action is thatcontinuous positive airway pressure acts as a pneumatic splint and mayprevent upper airway occlusion, such as by pushing the soft palate andtongue forward and away from the posterior oropharyngeal wall. Treatmentof OSA by CPAP therapy may be voluntary, and hence patients may electnot to comply with therapy if they find devices used to provide suchtherapy one or more of: uncomfortable, difficult to use, expensive andaesthetically unappealing.

Non-invasive ventilation (NIV) provides ventilatory support to a patientthrough the upper airways to assist the patient breathing and/ormaintain adequate oxygen levels in the body by doing some or all of thework of breathing. The ventilatory support is provided via anon-invasive patient interface. NIV has been used to treat CSR andrespiratory failure, in forms such as OHS, COPD, NMD and Chest Walldisorders.

Invasive ventilation (IV) provides ventilatory support to patients thatare no longer able to effectively breathe themselves and may be providedusing a tracheostomy tube.

In some forms, the comfort and effectiveness of these therapies may beimproved.

2.2.3 Treatment Systems

These therapies may be provided by a treatment system or device. Suchsystems and devices may also be used to screen, diagnose, or monitor acondition without treating it.

A treatment system may comprise a Respiratory Pressure Therapy Device(RPT device), an air circuit, a humidifier, a patient interface, anddata management.

2.2.3.1 Respiratory Pressure Therapy (RPT) Device

A respiratory pressure therapy (RPT) device may be used individually oras part of a system to deliver one or more of a number of therapiesdescribed above, such as by operating the device to generate a flow ofair for delivery to an interface to the airways. The flow of air may bepressurised. RPT devices typically comprise a pressure generator, suchas a motor-driven blower or a compressed gas reservoir, and areconfigured to supply a flow of air to the airway of a patient. In somecases, the flow of air may be supplied to the airway of the patient atpositive pressure. The outlet of the RPT device is connected via an aircircuit to a patient interface such as those described below.

One known RPT device used for treating sleep disordered breathing is theS9 Sleep Therapy System, manufactured by ResMed Limited. Another exampleof an RPT device is a ventilator. The ResMed Elisée™ 150 ventilator andResMed VS III™ ventilator may provide support for invasive andnon-invasive dependent ventilation suitable for adult or paediatricpatients for treating a number of conditions. These ventilators providevolumetric and barometric ventilation modes with a single or double limbcircuit.

2.2.3.2 Patient Interface

A patient interface may be used to interface respiratory equipment toits wearer, for example by providing a flow of air to an entrance to theairways. The flow of air may be provided via a mask to the nose and/ormouth, a tube to the mouth or a tracheostomy tube to the trachea of apatient. Depending upon the therapy to be applied, the patient interfacemay form a seal, e.g., with a region of the patient's face, tofacilitate the delivery of gas at a pressure at sufficient variance withambient pressure to effect therapy, e.g., at a positive pressure ofabout 10 cmH₂O relative to ambient pressure. For other forms of therapy,such as the delivery of oxygen, the patient interface may not include aseal sufficient to facilitate delivery to the airways of a supply of gasat a positive pressure of about 10 cmH₂O.

2.2.3.3 Humidifier

Delivery of a flow of air without humidification may cause drying ofairways. The use of a humidifier with an RPT device and the patientinterface produces humidified gas that minimizes drying of the nasalmucosa and increases patient airway comfort. In addition in coolerclimates, warm air applied generally to the face area in and about thepatient interface is more comfortable than cold air.

2.2.3.4 Data Management

There may be clinical reasons to obtain data to determine whether thepatient prescribed with respiratory therapy has been “compliant”, e.g.that the patient has used their RPT device according to one or more“compliance rules”. One example of a compliance rule for CPAP therapy isthat a patient, in order to be deemed compliant, is required to use theRPT device for at least four hours a night for at least 21 of 30consecutive days. In order to determine a patient's compliance, aprovider of the RPT device, such as a health care provider, may manuallyobtain data describing the patient's therapy using the RPT device,calculate the usage over a predetermined time period, and compare withthe compliance rule. Once the health care provider has determined thatthe patient has used their RPT device according to the compliance rule,the health care provider may notify a third party that the patient iscompliant.

There may be other aspects of a patient's therapy that would benefitfrom communication of therapy data to a third party or external system.

2.2.3.5 Vent Technologies

Some forms of treatment systems may include a vent to allow the washoutof exhaled carbon dioxide. The vent may allow a flow of gas from aninterior space of a patient interface, e.g., the plenum chamber, to anexterior of the patient interface, e.g., to ambient.

3 BRIEF SUMMARY OF THE TECHNOLOGY

The present technology is directed towards providing medical devicesused in the screening, diagnosis, monitoring, amelioration, treatment,or prevention of respiratory disorders having one or more of improvedcomfort, cost, efficacy, ease of use and manufacturability.

A first aspect of the present technology relates to apparatus used inthe screening, diagnosis, monitoring, amelioration, treatment orprevention of a respiratory disorder.

Another aspect of the present technology relates to methods used in thescreening, diagnosis, monitoring, amelioration, treatment or preventionof a respiratory disorder.

One form of the present technology comprises apparatus for treatment ofa respiratory disorder using a safety volume servo-ventilation mode, inwhich the gain of the servo-control of pressure support is based on adifferential between estimated inspiratory and expiratory volumes.

According to one aspect of the present technology, there is providedapparatus for treating a respiratory disorder in a patient. Theapparatus comprises a pressure generator configured to generate a flowof air so as to provide ventilatory support to the patient; a transducerconfigured to generate a signal representing a property of the flow ofair; and a controller configured to analyse the signal to estimate aninspiratory volume and an expiratory volume of a breath of the patientand servo-control the degree of ventilatory support to adjust anestimated tidal volume toward a target tidal volume. A gain of theservo-control is dependent on a difference between the estimatedinspiratory volume and the estimated expiratory volume.

In an example of this aspect, the gain decreases as the differencebetween the estimated inspiratory volume and the estimated expiratoryvolume increases. In a further example, the gain is dependent on thedifference between the estimated inspiratory volume and the estimatedexpiratory volume relative to the estimated tidal volume. Further still,the gain is dependent on the absolute magnitude of the differencebetween the estimated inspiratory volume and the estimated expiratoryvolume.

In another example of this aspect, the degree of the ventilatory supportis a pressure support of the ventilatory support.

According to a further aspect of the present technology, there isprovided a method of operating a respiratory treatment apparatusconfigured to generate a flow of air so as to provide ventilatorysupport to a patient. The method comprises measuring a property of theflow of air, using a transducer; analysing, in a controller, themeasured property to estimate the inspiratory volume and the expiratoryvolume of a breath of the patient; calculating, in a controller, a gaindependent on a difference between the estimated inspiratory volume andthe estimated expiratory volume; and servo-controlling, by a controller,the respiratory treatment apparatus using the calculated gain to adjustan estimated tidal volume for the patient toward a target tidal volume.

In an example of this aspect, the gain is dependent on the differencebetween the estimated inspiratory volume and the estimated expiratoryvolume relative to the estimated tidal volume. In other examples, thegain is dependent on the absolute magnitude of the difference betweenthe estimated inspiratory volume and the estimated expiratory volume. Inother examples, the degree of the ventilatory support is a pressuresupport of the ventilatory support.

According to a further aspect of the present technology, there isprovided a system for treating a respiratory disorder in a patient. Thesystem comprises means for generating a flow of air so as to provideventilatory support to the patient; means for generating a signalrepresenting a property of the flow of air; means for analysing thesignal to estimate the inspiratory volume and the expiratory volume of abreath of the patient; and means for servo-controlling the degree ofventilatory support to adjust an estimated tidal volume toward a targettidal volume. A gain of the servo-control is dependent on a differencebetween the estimated inspiratory volume and the estimated expiratoryvolume.

According to a further aspect of the present technology, an apparatusfor treating a respiratory disorder in a patient is provided. Theapparatus comprises a blower configured to deliver a supply of air thatprovides ventilatory support to the patient; a transducer configured togenerate a signal representing a property of the supply of air; and acontroller. The controller is configured to analyse the signal toestimate an inspiratory volume and an expiratory volume of a breath ofthe patient; and adjust servo-control gain based on a difference betweenthe estimated inspiratory volume and the estimated expiratory volume.

In an example of this aspect, the controller adjusts the servo-controlgain so that the gain decreases as the difference between the estimatedinspiratory volume and the estimated expiratory volume increases. Inanother example, an adjustment in servo-control gain causes a reductionin a rate of adjustment of pressure support of the supply of air. Inother examples, the controller is configured to adjust the servo-controlgain when the difference is greater than or equal to 20%, or furtherconfigured to adjust the servo-control gain to a default value when thedifference returns to a value that is less than 20%.

The methods, systems, devices and apparatus described may be implementedso as to improve the functionality of a processor, such as a processorof a specific purpose computer, respiratory monitor and/or a respiratorytherapy apparatus. Moreover, the described methods, systems, devices andapparatus can provide improvements in the technological field ofautomated management, monitoring and/or treatment of respiratoryconditions, including, for example, sleep disordered breathing.

Of course, portions of the aspects may form sub-aspects of the presenttechnology. Also, various ones of the sub-aspects, aspects and/orexample may be combined in various manners and also constituteadditional aspects or sub-aspects of the present technology.

Other features of the technology will be apparent from consideration ofthe information contained in the following detailed description,abstract, drawings and claims.

4 BRIEF DESCRIPTION OF THE DRAWINGS

The present technology is illustrated by way of example, and not by wayof limitation, in the figures of the accompanying drawings, in whichlike reference numerals refer to similar elements including:

4.1 Treatment Systems

FIG. 1 shows a system including a patient 1000 wearing a patientinterface 3000, in the form of a full-face mask, receiving a supply ofair at positive pressure from an RPT device 4000. Air from the RPTdevice is humidified in a humidifier 5000, and passes along an aircircuit 4170 to the patient 1000. The patient is sleeping in a sidesleeping position.

4.2 Respiratory System and Facial Anatomy

FIG. 2 shows an overview of a human respiratory system including thenasal and oral cavities, the larynx, vocal folds, oesophagus, trachea,bronchus, lung, alveolar sacs, heart and diaphragm.

4.3 Patient Interface

FIG. 3 shows a patient interface in the form of a nasal mask inaccordance with one form of the present technology.

4.4 RPT Device

FIG. 4A shows an RPT device in accordance with one form of the presenttechnology.

FIG. 4B is a schematic diagram of the pneumatic path of an RPT device inaccordance with one form of the present technology. The directions ofupstream and downstream are indicated with reference to the blower andthe patient interface. The blower is defined to be upstream of thepatient interface and the patient interface is defined to be downstreamof the blower, regardless of the actual flow direction at any particularmoment. Items which are located within the pneumatic path between theblower and the patient interface are downstream of the blower andupstream of the patient interface.

FIG. 4C is a schematic diagram of the electrical components of an RPTdevice in accordance with one form of the present technology.

FIG. 4D is a schematic diagram of the algorithms implemented in an RPTdevice in accordance with one form of the present technology.

4.5 Humidifier

FIG. 5A shows an isometric view of a humidifier in accordance with oneform of the present technology.

FIG. 5B shows an isometric view of a humidifier in accordance with oneform of the present technology, showing a humidifier reservoir 5110removed from the humidifier reservoir dock 5130.

4.6 Breathing Waveforms

FIG. 6 shows a model typical breath waveform of a person while sleeping.

4.7 Respiratory Pressure Therapy Modes

FIG. 7A is a graph illustrating undesirable behaviour of pressuresupport during sudden leak changes in conventional safety volume mode.

FIG. 7B is a graph illustrating behaviour of pressure support duringsudden leak changes in safety volume mode according to one form of thepresent technology.

FIG. 8 is a graph illustrating of the adjustment of the servo-controlgain in safety volume mode as a function of a relative differentialbetween inspiratory and expiratory volumes.

FIG. 9 is a flow chart that provides an overview of method for adjustingservo-controller gain in a respiratory apparatus.

5 DETAILED DESCRIPTION OF EXAMPLES OF THE TECHNOLOGY

Before the present technology is described in further detail, it is tobe understood that the technology is not limited to the particularexamples described herein, which may vary. It is also to be understoodthat the terminology used in this disclosure is for the purpose ofdescribing only the particular examples discussed herein, and is notintended to be limiting.

The following description is provided in relation to various exampleswhich may share one or more common characteristics and/or features. Itis to be understood that one or more features of any one example may becombinable with one or more features of another example or otherexamples. In addition, any single feature or combination of features inany of the examples may constitute a further example.

5.1 Therapy

In one form, the present technology comprises a method for treating arespiratory disorder comprising the step of applying positive pressureto the entrance of the airways of a patient 1000.

In certain examples of the present technology, a supply of air atpositive pressure is provided to the nasal passages of the patient viaone or both nares.

In certain examples of the present technology, mouth breathing islimited, restricted or prevented.

5.2 Treatment Systems

In one form, the present technology comprises an apparatus or device fortreating a respiratory disorder. The apparatus or device may comprise anRPT device 4000 for supplying pressurised air to the patient 1000 via anair circuit 4170 to a patient interface 3000, as shown for example inFIG. 1.

5.3 Patient Interface

A non-invasive patient interface 3000 in accordance with one aspect ofthe present technology comprises the following functional aspects: aseal-forming structure 3100, a plenum chamber 3200, a positioning andstabilising structure 3300, a vent 3400, one form of connection port3600 for connection to air circuit 4170, and a forehead support 3700,such as depicted for example in FIG. 3. In some forms a functionalaspect may be provided by one or more physical components. In someforms, one physical component may provide one or more functionalaspects. In use the seal-forming structure 3100 is arranged to surroundan entrance to the airways of the patient so as to facilitate the supplyof air at positive pressure to the airways.

5.4 RPT Device

An RPT device 4000 in accordance with one aspect of the presenttechnology comprises mechanical, pneumatic, and/or electrical componentsand is configured to execute one or more algorithms 4300, such as any ofthe methods, in whole or in part, described herein. The RPT device 4000may be configured to generate a flow of air for delivery to a patient'sairways, such as to treat one or more of the respiratory conditionsdescribed elsewhere in the present document. FIGS. 4A through 4D provideillustrative examples of the components or schematics that may compriseRPT device 4000.

The RPT device may have an external housing 4010, formed in two parts,an upper portion 4012 and a lower portion 4014. Furthermore, theexternal housing 4010 may include one or more panel(s) 4015. The RPTdevice 4000 comprises a chassis 4016 that supports one or more internalcomponents of the RPT device 4000. The RPT device 4000 may include ahandle 4018.

The pneumatic path of the RPT device 4000 may comprise one or more airpath items, e.g., an inlet air filter 4112, an inlet muffler 4122, apressure generator 4140 capable of supplying air at positive pressure(e.g., a blower 4142), an outlet muffler 4124 and one or moretransducers 4270, such as pressure sensors 4272 and flow rate sensors4274.

One or more of the air path items may be located within a removableunitary structure which will be referred to as a pneumatic block 4020.The pneumatic block 4020 may be located within the external housing4010. In one form a pneumatic block 4020 is supported by, or formed aspart of the chassis 4016.

The RPT device 4000 may have an electrical power supply 4210, one ormore input devices 4220, a central controller 4230, a therapy devicecontroller 4240, a pressure generator 4140, one or more protectioncircuits 4250, memory 4260, transducers 4270, data communicationinterface 4280 and one or more output devices 4290. Electricalcomponents 4200 may be mounted on a single Printed Circuit BoardAssembly (PCBA) 4202. In an alternative form, the RPT device 4000 mayinclude more than one PCBA 4202.

5.4.1 RPT Device Mechanical & Pneumatic Components

An RPT device may comprise one or more of the following components in anintegral unit. In an alternative form, one or more of the followingcomponents may be located as respective separate units.

5.4.1.1 Air Filter(s)

An RPT device in accordance with one form of the present technology mayinclude an air filter 4110, or a plurality of air filters 4110.

In one form, an inlet air filter 4112 is located at the beginning of thepneumatic path upstream of a pressure generator 4140.

In one form, an outlet air filter 4114, for example an antibacterialfilter, is located between an outlet of the pneumatic block 4020 and apatient interface 3000.

5.4.1.2 Muffler(s)

An RPT device in accordance with one form of the present technology mayinclude a muffler 4120, or a plurality of mufflers 4120.

In one form of the present technology, an inlet muffler 4122 is locatedin the pneumatic path upstream of a pressure generator 4140.

In one form of the present technology, an outlet muffler 4124 is locatedin the pneumatic path between the pressure generator 4140 and a patientinterface 3000.

5.4.1.3 Pressure Generator

In one form of the present technology, a pressure generator 4140 forproducing a flow, or a supply, of air at positive pressure is acontrollable blower 4142. For example the blower 4142 may include abrushless DC motor 4144 with one or more impellers housed in a blowerhousing, such as in a volute. The blower may be capable of delivering asupply of air, for example at a rate of up to about 120 litres/minute,at a positive pressure in a range from about 4 cmH₂O to about 20 cmH₂O,or in other forms up to about 30 cmH₂O. The blower may be as describedin any one of the following patents or patent applications the contentsof which are incorporated herein by reference in their entirety: U.S.Pat. Nos. 7,866,944; 8,638,014; 8,636,479; and PCT Patent ApplicationPublication No. WO 2013/020167.

The pressure generator 4140 is under the control of the therapy devicecontroller 4240.

In other forms, a pressure generator 4140 may be a piston-driven pump, apressure regulator connected to a high pressure source (e.g. compressedair reservoir), or a bellows.

5.4.1.4 Transducer(s)

Transducers may be internal of the RPT device, or external of the RPTdevice. External transducers may be located for example on or form partof the air circuit, e.g., the patient interface. External transducersmay be in the form of non-contact sensors such as a Doppler radarmovement sensor that transmit or transfer data to the RPT device.

In one form of the present technology, one or more transducers 4270 arelocated upstream and/or downstream of the pressure generator 4140. Theone or more transducers 4270 may be constructed and arranged to generatesignals representing properties of the flow of air such as a flow rate,a pressure or a temperature at that point in the pneumatic path.

In one form of the present technology, one or more transducers 4270 maybe located proximate to the patient interface 3000.

In one form, a signal from a transducer 4270 may be filtered, such as bylow-pass, high-pass or band-pass filtering.

5.4.1.4.1 Flow Rate Sensor

A flow rate sensor 4274 in accordance with the present technology may bebased on a differential pressure transducer, for example, an SDP600Series differential pressure transducer from SENSIRION.

In one form, a signal representing a flow rate from the flow rate sensor4274 is received by the central controller 4230.

5.4.1.4.2 Pressure Sensor

A pressure sensor 4272 in accordance with the present technology islocated in fluid communication with the pneumatic path. An example of asuitable pressure sensor is a transducer from the HONEYWELL ASDX series.An alternative suitable pressure sensor is a transducer from the NPASeries from GENERAL ELECTRIC.

In one form, a signal from the pressure sensor 4272 is received by thecentral controller 4230.

5.4.1.4.3 Motor Speed Transducer

In one form of the present technology a motor speed transducer 4276 isused to determine a rotational velocity of the motor 4144 and/or theblower 4142. A motor speed signal from the motor speed transducer 4276may be provided to the therapy device controller 4240. The motor speedtransducer 4276 may, for example, be a speed sensor, such as a Halleffect sensor.

5.4.1.5 Anti-Spill Back Valve

In one form of the present technology, an anti-spill back valve 4160 islocated between the humidifier 5000 and the pneumatic block 4020. Theanti-spill back valve is constructed and arranged to reduce the riskthat water will flow upstream from the humidifier 5000, for example tothe motor 4144.

5.4.2 RPT Device Electrical Components 5.4.2.1 Power Supply

A power supply 4210 may be located internal or external of the externalhousing 4010 of the RPT device 4000.

In one form of the present technology, power supply 4210 provideselectrical power to the RPT device 4000 only. In another form of thepresent technology, power supply 4210 provides electrical power to bothRPT device 4000 and humidifier 5000.

5.4.2.2 Input Devices

In one form of the present technology, an RPT device 4000 includes oneor more input devices 4220 in the form of buttons, switches or dials toallow a person to interact with the device. The buttons, switches ordials may be physical devices, or software devices accessible via atouch screen. The buttons, switches or dials may, in one form, bephysically connected to the external housing 4010, or may, in anotherform, be in wireless communication with a receiver that is in electricalconnection to the central controller 4230.

In one form, the input device 4220 may be constructed and arranged toallow a person to select a value and/or a menu option.

5.4.2.3 Central Controller

In one form of the present technology, the central controller 4230 isone or a plurality of processors suitable to control an RPT device 4000.

Suitable processors may include an x86 INTEL processor, a processorbased on ARM® Cortex®-M processor from ARM Holdings such as an STM32series microcontroller from ST MICROELECTRONIC. In certain alternativeforms of the present technology, a 32-bit RISC CPU, such as an STR9series microcontroller from ST MICROELECTRONICS or a 16-bit RISC CPUsuch as a processor from the MSP430 family of microcontrollers,manufactured by TEXAS INSTRUMENTS may also be suitable.

In one form of the present technology, the central controller 4230 is adedicated electronic circuit.

In one form, the central controller 4230 is an application-specificintegrated circuit. In another form, the central controller 4230comprises discrete electronic components.

The central controller 4230 may be configured to receive input signal(s)from one or more transducers 4270, one or more input devices 4220, andthe humidifier 5000.

The central controller 4230 may be configured to provide outputsignal(s) to one or more of an output device 4290, a therapy devicecontroller 4240, a data communication interface 4280, and the humidifier5000.

In some forms of the present technology, the central controller 4230 isconfigured to implement the one or more methodologies described herein,such as the one or more algorithms 4300 expressed as computer programsstored in a non-transitory computer readable storage medium, such asmemory 4260. In some forms of the present technology, the centralcontroller 4230 may be integrated with an RPT device 4000. However, insome forms of the present technology, some methodologies may beperformed by a remotely located device. For example, the remotelylocated device may determine control settings for a ventilator or detectrespiratory related events by analysis of stored data such as from anyof the sensors described herein.

5.4.2.4 Clock

The RPT device 4000 may include a clock 4232 that is connected to thecentral controller 4230.

5.4.2.5 Therapy Device Controller

In one form of the present technology, therapy device controller 4240 isa therapy control module 4330 that forms part of the algorithms 4300executed by the central controller 4230.

In one form of the present technology, therapy device controller 4240 isa dedicated motor control integrated circuit. For example, in one form aMC33035 brushless DC motor controller, manufactured by ONSEMI is used.

5.4.2.6 Protection Circuits

The one or more protection circuits 4250 in accordance with the presenttechnology may comprise an electrical protection circuit, a temperatureand/or pressure safety circuit.

5.4.2.7 Memory

In accordance with one form of the present technology the RPT device4000 includes memory 4260, e.g., non-volatile memory. In some forms,memory 4260 may include battery powered static RAM. In some forms,memory 4260 may include volatile RAM.

Memory 4260 may be located on the PCBA 4202. Memory 4260 may be in theform of EEPROM, or NAND flash.

Additionally or alternatively, RPT device 4000 includes a removable formof memory 4260, for example a memory card made in accordance with theSecure Digital (SD) standard.

In one form of the present technology, the memory 4260 acts as anon-transitory computer readable storage medium on which is storedcomputer program instructions expressing the one or more methodologiesdescribed herein, such as the one or more algorithms 4300.

5.4.2.8 Data Communication Systems

In one form of the present technology, a data communication interface4280 is provided, and is connected to the central controller 4230. Datacommunication interface 4280 may be connectable to a remote externalcommunication network 4282 and/or a local external communication network4284. The remote external communication network 4282 may be connectableto a remote external device 4286. The local external communicationnetwork 4284 may be connectable to a local external device 4288.

In one form, data communication interface 4280 is part of the centralcontroller 4230. In another form, data communication interface 4280 isseparate from the central controller 4230, and may comprise anintegrated circuit or a processor.

In one form, remote external communication network 4282 is the Internet.The data communication interface 4280 may use wired communication (e.g.via Ethernet, or optical fibre) or a wireless protocol (e.g. CDMA, GSM,LTE) to connect to the Internet.

In one form, local external communication network 4284 utilises one ormore communication standards, such as Bluetooth, or a consumer infraredprotocol.

In one form, remote external device 4286 is one or more computers, forexample a cluster of networked computers. In one form, remote externaldevice 4286 may be virtual computers, rather than physical computers. Ineither case, such a remote external device 4286 may be accessible to anappropriately authorised person such as a clinician.

The local external device 4288 may be a personal computer, mobile phone,tablet or remote control.

5.4.2.9 Output Devices Including Optional Display, Alarms

An output device 4290 in accordance with the present technology may takethe form of one or more of a visual, audio and haptic unit. A visualdisplay may be a Liquid Crystal Display (LCD) or Light Emitting Diode(LED) display.

5.4.2.9.1 Display Driver

A display driver 4292 receives as an input the characters, symbols, orimages intended for display on the display 4294, and converts them tocommands that cause the display 4294 to display those characters,symbols, or images.

5.4.2.9.2 Display

A display 4294 is configured to visually display characters, symbols, orimages in response to commands received from the display driver 4292.For example, the display 4294 may be an eight-segment display, in whichcase the display driver 4292 converts each character or symbol, such asthe figure “0”, to eight logical signals indicating whether the eightrespective segments are to be activated to display a particularcharacter or symbol.

5.4.3 RPT Device Algorithms

As mentioned above, in some forms of the present technology, the centralcontroller 4230 may be configured to implement one or more algorithms4300 expressed as computer programs stored in a non-transitory computerreadable storage medium, such as memory 4260. The algorithms 4300 aregenerally grouped into groups referred to as modules.

5.4.3.1 Pre-Processing Module

A pre-processing module 4310 in accordance with one form of the presenttechnology receives as an input a signal from a transducer 4270, forexample a flow rate sensor 4274 or pressure sensor 4272, and performsone or more process steps to calculate one or more output values thatwill be used as an input to another module, for example a therapy enginemodule 4320, and may be depicted as shown for example in FIG. 4D.

In one form of the present technology, the output values include theinterface or mask pressure Pm, the respiratory flow rate Qr, and theleak flow rate Ql.

In various forms of the present technology, the pre-processing module4310 comprises one or more of the following algorithms: pressurecompensation 4312, vent flow rate estimation 4314, leak flow rateestimation 4316, and respiratory flow rate estimation 4318.

5.4.3.1.1 Pressure Compensation

In one form of the present technology, a pressure compensation algorithm4312 receives as an input a signal indicative of the pressure in thepneumatic path proximal to an outlet of the pneumatic block. Thepressure compensation algorithm 4312 estimates the pressure drop throughthe air circuit 4170 and provides as an output an estimated pressure,Pm, in the patient interface 3000.

5.4.3.1.2 Vent Flow Rate Estimation

In one form of the present technology, a vent flow rate estimationalgorithm 4314 receives as an input an estimated pressure, Pm, in thepatient interface 3000 and estimates a vent flow rate of air, Qv, from avent 3400 in a patient interface 3000.

5.4.3.1.3 Leak Flow Rate Estimation

In one form of the present technology, a leak flow rate estimationalgorithm 4316 receives as an input a total flow rate, Qt, and a ventflow rate Qv, and provides as an output an estimate of the leak flowrate Ql. In one form, the leak flow rate estimation algorithm estimatesthe leak flow rate Ql by calculating an average of the differencebetween total flow rate Qt and vent flow rate Qv over a periodsufficiently long to include several breathing cycles, e.g. about 10seconds.

In one form, the leak flow rate estimation algorithm 4316 receives as aninput a total flow rate Qt, a vent flow rate Qv, and an estimatedpressure, Pm, in the patient interface 3000, and provides as an output aleak flow rate Ql, by calculating a leak conductance, and determining aleak flow rate Ql to be a function of leak conductance and pressure, Pm.Leak conductance is calculated as the quotient of low pass filterednon-vent flow rate equal to the difference between total flow rate Qtand vent flow rate Qv, and low pass filtered square root of pressure Pm,where the low pass filter time constant has a value sufficiently long toinclude several breathing cycles, e.g. about 10 seconds. The leak flowrate Ql may be estimated as the product of leak conductance and afunction of pressure, Pm.

A sudden change in leak is a change on a time scale that is shorter thanthe leak flow rate estimation algorithm 4316 can initially keep up with,i.e. of the order of a breathing cycle or less. In certain therapy modessuch sudden changes in leak need special handling, as described below.

5.4.3.1.4 Respiratory Flow Rate Estimation

In one form of the present technology, a respiratory flow rateestimation algorithm 4318 receives as an input a total flow rate, Qt, avent flow rate, Qv, and a leak flow rate, Ql, and estimates arespiratory flow rate of air, Qr, to the patient, by subtracting thevent flow rate Qv and the leak flow rate Ql from the total flow rate Qt.

5.4.3.2 Therapy Engine Module

In one form of the present technology, a therapy engine module 4320receives as inputs one or more of a pressure, Pm, in a patient interface3000, and a respiratory flow rate of air to a patient, Qr, and providesas an output one or more therapy parameters.

In one form of the present technology, a therapy parameter is atreatment pressure Pt.

In one form of the present technology, therapy parameters are one ormore of an amplitude of a pressure variation, a base pressure, and atarget tidal volume.

In various forms, the therapy engine module 4320 comprises one or moreof the following algorithms: phase determination 4321, waveformdetermination 4322, tidal volume estimation 4323, and therapy parameterdetermination 4329.

5.4.3.2.1 Phase Determination

In one form of the present technology, the RPT device 4000 may determinebreathing phase.

In one form of the present technology, a phase determination algorithm4321 receives as an input a signal indicative of respiratory flow rate,Qr, and provides as an output a phase Φ of a current breathing cycle ofa patient 1000.

In some forms, known as discrete phase determination, the phase output Φis a discrete variable. One implementation of discrete phasedetermination provides a bi-valued phase output Φ with values of eitherinhalation or exhalation, for example represented as values of 0 and 0.5revolutions respectively, upon detecting the start of spontaneousinhalation and exhalation respectively. RPT devices 4000 that “trigger”and “cycle” effectively perform discrete phase determination, since thetrigger and cycle points are the instants at which the phase changesfrom exhalation to inhalation and from inhalation to exhalation,respectively. In one implementation of bi-valued phase determination,the phase output Φ is determined to have a discrete value of 0 (thereby“triggering” the RPT device 4000) when the respiratory flow rate Qr hasa value that exceeds a positive threshold, and a discrete value of 0.5revolutions (thereby “cycling” the RPT device 4000) when a respiratoryflow rate Qr has a value that is more negative than a negativethreshold. The inhalation time Ti and the exhalation time Te may beestimated as typical values over many respiratory cycles of the timespent with phase Φ equal to 0 (indicating inspiration) and 0.5(indicating expiration) respectively.

Another implementation of discrete phase determination provides atri-valued phase output Φ with a value of one of inhalation,mid-inspiratory pause, and exhalation.

In other forms, known as continuous phase determination, the phaseoutput Φ is a continuous variable, for example varying from 0 to 1revolutions, or 0 to 2π radians. RPT devices 4000 that performcontinuous phase determination may trigger and cycle when the continuousphase reaches 0 and 0.5 revolutions, respectively. In one implementationof continuous phase determination, the phase Φ is first discretelyestimated from the respiratory flow rate Qr as described above, as arethe inhalation time Ti and the exhalation time Te. The continuous phaseΦ at any given instant may be determined as the half the proportion ofthe inhalation time Ti that has elapsed since the previous triggerinstant, or 0.5 revolutions plus half the proportion of the exhalationtime Te that has elapsed since the previous cycle instant (whicheverinstant was more recent).

5.4.3.2.2 Waveform Determination

In one form of the present technology, the therapy parameterdetermination algorithm 4329 provides an approximately constanttreatment pressure throughout a respiratory cycle of a patient.

In other forms of the present technology, the therapy control module4330 controls the pressure generator 4140 to provide a treatmentpressure Pt that varies as a function of phase Φ of a respiratory cycleof a patient according to a waveform template Π(Φ).

In one form of the present technology, a waveform determinationalgorithm 4322 provides a waveform template Π(Φ) with values in therange [0, 1] on the domain of phase values Φ provided by the phasedetermination algorithm 4321 to be used by the therapy parameterdetermination algorithm 4329.

In one form, suitable for either discrete or continuously-valued phase,the waveform template Π(Φ) is a square-wave template, having a value of1 for values of phase up to and including 0.5 revolutions, and a valueof 0 for values of phase above 0.5 revolutions. In one form, suitablefor continuously-valued phase, the waveform template Π(Φ) comprises twosmoothly curved portions, namely a smoothly curved (e.g. raised cosine)rise from 0 to 1 for values of phase up to 0.5 revolutions, and asmoothly curved (e.g. exponential) decay from 1 to 0 for values of phaseabove 0.5 revolutions. In one form, suitable for continuously-valuedphase, the waveform template Π(Φ) is based on a square wave, but with asmooth rise from 0 to 1 for values of phase up to a “rise time” that isless than 0.5 revolutions, and a smooth fall from 1 to 0 for values ofphase within a “fall time” after 0.5 revolutions, with a “fall time”that is less than 0.5 revolutions.

In some forms of the present technology, the waveform determinationalgorithm 4322 selects a waveform template Π(Φ) from a library ofwaveform templates, dependent on a setting of the RPT device. Eachwaveform template Π(Φ) in the library may be provided as a lookup tableof values Π against phase values Φ. In other forms, the waveformdetermination algorithm 4322 computes a waveform template Π(Φ) “on thefly” using a predetermined functional form, possibly parametrised by oneor more parameters (e.g. time constant of an exponentially curvedportion). The parameters of the functional form may be predetermined ordependent on a current state of the patient 1000.

In some forms of the present technology, suitable for discrete bi-valuedphase of either inhalation (Φ=0 revolutions) or exhalation (Φ=0.5revolutions), the waveform determination algorithm 4322 computes awaveform template Π “on the fly” as a function of both discrete phase Φand time t measured since the most recent trigger instant. In one suchform, the waveform determination algorithm 4322 computes the waveformtemplate Π(Φ, t) in two portions (inspiratory and expiratory) asfollows:

${\Pi \left( {\Phi,t} \right)} = \left\{ \begin{matrix}{{\Pi_{i}(t)},} & {\Phi = 0} \\{{\Pi_{e}\left( {t - T_{i}} \right)},} & {\Phi = 0.5}\end{matrix} \right.$

where Π_(i)(t) and Π_(e)(t) are inspiratory and expiratory portions ofthe waveform template Π(Φ, t). In one such form, the inspiratory portionΠ1 _(i)(t) of the waveform template is a smooth rise from 0 to 1parametrised by a rise time, and the expiratory portion Π_(e)(t) of thewaveform template is a smooth fall from 1 to 0 parametrised by a falltime.

5.4.3.2.3 Tidal Volume Estimation

In some forms of the present technology, the central controller 4230executes one or more tidal volume estimation algorithms 4323 for theestimation of tidal volume using the values returned by one or more ofthe other algorithms in the therapy engine module 4320.

In one form of the present technology, the tidal volume estimationalgorithm 4323 receives as an input a signal indicative of respiratoryflow rate Qr and the phase Φ determined by the phase determinationalgorithm 4321, and returns an estimate of the tidal volume V_(T) of themost recent breath. The tidal volume V_(T) may be estimated as theinspiratory (tidal) volume Vi for the breath, the expiratory (tidal)volume Ve for the breath, or some combination of the two, e.g. the meanor average. The inspiratory volume Vi may be estimated as the integralof the respiratory flow rate Qr over the inspiratory portion of thebreath (indicated by phase Φ being less than 0.5). The expiratory volumeVe may be estimated as the integral of the respiratory flow rate Qr overthe expiratory portion of the breath (indicated by phase Φ being greaterthan or equal to 0.5).

5.4.3.2.4 Determination of Therapy Parameters

In some forms of the present technology, the central controller 4230executes one or more therapy parameter determination algorithms 4329 forthe determination of one or more therapy parameters using the valuesreturned by one or more of the other algorithms in the therapy enginemodule 4320.

In one form of the present technology, the therapy parameter is aninstantaneous treatment pressure Pt. In one implementation of this form,the therapy parameter determination algorithm 4329 determines thetreatment pressure Pt using the equation

Pt=AΠ(Φ,t)+P ₀  (1)

where:

-   -   A is the amplitude,    -   Π(Φ, t) is the waveform template value (in the range 0 to 1) at        the current value Φ of phase and t of time, and    -   P₀ is a base pressure.

If the waveform determination algorithm 4322 provides the waveformtemplate Π(Φ, t) as a lookup table of values Π indexed by phase Φ, thetherapy parameter determination algorithm 4329 applies equation (1) bylocating the nearest lookup table entry to the current value Φ of phasereturned by the phase determination algorithm 4321, or by interpolationbetween the two entries straddling the current value Φ of phase.

The values of the amplitude A and the base pressure P₀ may be set by thetherapy parameter determination algorithm 4329 depending on the chosenrespiratory pressure therapy mode in the manner described below.

5.4.3.3 Therapy Control Module

The therapy control module 4330 in accordance with one aspect of thepresent technology receives as inputs the therapy parameters from thetherapy parameter determination algorithm 4329 of the therapy enginemodule 4320, and controls the pressure generator 4140 to deliver a flowof air in accordance with the therapy parameters.

In one form of the present technology, the therapy parameter is atreatment pressure Pt, and the therapy control module 4330 controls thepressure generator 4140 to deliver a flow of air whose mask pressure Pmat the patient interface 3000 is equal to the treatment pressure Pt.

5.4.3.4 Detection of Fault Conditions

In one form of the present technology, the central controller 4230executes one or more methods 4340 for the detection of fault conditions.The fault conditions detected by the one or more methods 4340 mayinclude at least one of the following:

-   -   Power failure (no power, or insufficient power)    -   Transducer fault detection    -   Failure to detect the presence of a component    -   Operating parameters outside recommended ranges (e.g. pressure,        flow rate, temperature, PaO₂)    -   Failure of a test alarm to generate a detectable alarm signal.

Upon detection of the fault condition, the corresponding algorithm 4340signals the presence of the fault by one or more of the following:

-   -   Initiation of an audible, visual &/or kinetic (e.g. vibrating)        alarm    -   Sending a message to an external device    -   Logging of the incident

5.5 Air Circuit

An air circuit 4170 in accordance with an aspect of the presenttechnology is a conduit or a tube constructed and arranged to allow, inuse, a flow of air to travel between two components such as the RPTdevice 4000 and the patient interface 3000.

In particular, the air circuit 4170 may be in fluid connection with theoutlet of the pneumatic block 4020 and the patient interface. The aircircuit may be referred to as an air delivery tube. In some cases, theremay be separate limbs of the circuit for inhalation and exhalation. Inother cases, a single limb air circuit is used.

5.6 Humidifier

In one form of the present technology there is provided a humidifier5000 (e.g. as shown in FIG. 5A) to change the absolute humidity of airor gas for delivery to a patient relative to ambient air. Typically, thehumidifier 5000 is used to increase the absolute humidity and increasethe temperature of the flow of air (relative to ambient air) beforedelivery to the patient's airways.

The humidifier 5000 may comprise a humidifier reservoir 5110, ahumidifier inlet 5002 to receive a flow of air, and a humidifier outlet5004 to deliver a humidified flow of air. In some forms, as shown inFIG. 5A and FIG. 5B, an inlet and an outlet of the humidifier reservoir5110 may be the humidifier inlet 5002 and the humidifier outlet 5004respectively. The humidifier 5000 may further comprise a humidifier base5006, which may be adapted to receive the humidifier reservoir 5110 andcomprise a heating element 5240.

5.7 Breathing Waveforms

FIG. 6 shows a model typical breath waveform of a person while sleeping.The horizontal axis is time, and the vertical axis is respiratory flowrate. While the parameter values may vary, a typical breath may have thefollowing approximate values: tidal volume V_(T) 0.5 L, inhalation timeTi 1.6 s, peak inspiratory flow rate Qpeak 0.4 L/s, exhalation time Te2.4 s, peak expiratory flow rate Qpeak −0.5 L/s. The total duration ofthe breath Ttot is about 4 s. The person typically breathes at a rate ofabout 15 breaths per minute (BPM), with Ventilation Vent about 7.5L/min. A typical duty cycle, the ratio of Ti to Ttot, is about 40%.

5.8 Respiratory Pressure Therapy Modes

Various respiratory pressure therapy modes may be implemented by the RPTdevice 4000 depending on the values of the parameters A and P₀ in thetreatment pressure equation (1) used by the therapy parameterdetermination algorithm 4329 in one form of the present technology.

In some implementations of this form of the present technology, theamplitude A in the treatment pressure equation (1) is identically zero,so the treatment pressure Pt is identically equal to the base pressureP₀ throughout the respiratory cycle. Such implementations are generallygrouped under the heading of CPAP therapy. In other implementations ofthis form of the present technology, the value of amplitude A inequation (1) is positive. Such implementations are known as bi-leveltherapy, because in determining the treatment pressure Pt using equation(1) with positive amplitude A, the therapy parameter determinationalgorithm 4329 oscillates the treatment pressure Pt between two valuesor levels in synchrony with the spontaneous respiratory effort of thepatient 1000. That is, based on the typical waveform templates MD, t)described above, the therapy parameter determination algorithm 4329increases the treatment pressure Pt to P₀+A (known as the IPAP) at thestart of, or during, or inspiration and decreases the treatment pressurePt to the base pressure P₀ (known as the EPAP) at the start of, orduring, expiration.

In some forms of bi-level therapy, the amplitude A is large enough thatthe RPT device 4000 does some or all of the work of breathing of thepatient 1000. Such forms may be referred to as providing ventilatorysupport to the patient 1000. In such forms, known as pressure support orpressure controlled ventilation therapy, the amplitude A is referred toas the pressure support, or swing. In pressure support ventilationtherapy, the IPAP is the base pressure P₀ plus the pressure support A,and the EPAP is the base pressure P₀.

In some forms of pressure support ventilation therapy, known as fixedpressure support ventilation therapy, the pressure support A is fixed ata predetermined value, e.g. 10 cmH₂O. The predetermined pressure supportvalue is a setting of the RPT device 4000, and may be set for example byhard-coding during configuration of the RPT device 4000 or by manualentry through the input device 4220.

In other forms of pressure support ventilation therapy, broadly known asservo-ventilation, the therapy parameter determination algorithm 4329takes as input some currently measured or estimated parameter of therespiratory cycle and a target value of that respiratory parameter, andcontinuously adjusts the parameters of equation (1) to adjust thecurrent measure of the respiratory parameter towards the target value.One example of servo-ventilation in which the respiratory parameter istidal volume V_(T) is sometimes referred to as safety volume mode.

In some forms of servo-ventilation, the therapy parameter determinationalgorithm 4329 applies a servo-control methodology to repeatedly computethe pressure support A so as to adjust the current measure of therespiratory parameter towards the target value. One such servo-controlmethodology is Proportional-Integral (PI) control. In one implementationof PI control for safety volume mode, an adjustment ΔA to the currentpressure support A is computed as:

$\begin{matrix}{{\Delta \; A} = {\frac{G}{C_{nom}}\left( {{V_{T}({target})} - V_{T}} \right)}} & (2)\end{matrix}$

where G is the servo-control gain, Cnom is a nominal compliance constant(which is typically set to 60 ml/cmH₂O for adults and to 40 ml/cmH₂O forpediatrics, but may be varied for different patient sub-types) andV_(T)(target) is the target tidal volume (in millilitres). The gain G istypically a constant value of one.

Other servo-control methodologies that may be applied by the therapyparameter determination algorithm 4329 include proportional (P),proportional-differential (PD), and proportional-integral-differential(PID).

The value of the pressure support A computed via equation (2) may beclipped to a range defined as [Amin, Amax]. In this implementation, thepressure support A sits by default at the minimum pressure support Aminuntil the measure of tidal volume V_(T) falls below the target tidalvolume V_(T)(target), at which point A starts increasing, only fallingback to Amin when V_(T) exceeds V_(T)(target) once again.

The pressure support limits Amin and Amax are settings of the RPT device4000, set for example by hard-coding during configuration of the RPTdevice 4000 or by manual entry through the input device 4220.

5.8.1 Sudden Leak Change Handling

Sudden change in leak (either the sudden appearance of a leak or itssudden resolution) may cause the estimate of leak flow rate Ql, andhence the respiratory flow rate estimate Qr, to be temporarilyinaccurate while the leak flow rate estimation algorithm 4316 “catchesup” with the sudden change. When the leak appears, the respiratory flowrate Qr is overestimated for a period lasting perhaps a few breaths.Then, when the leak is resolved, the respiratory flow rate Qr isunderestimated for a period. In addition, the phase determinationalgorithm 4321 may be caused to overestimate or underestimate theinspiratory portions of the breaths respectively. The result is thatwhen the leak appears, the inspiratory volume Vi tends to be temporarilyoverestimated and the expiratory volume Ve tends to be temporarilyunderestimated. When the leak is resolved, the expiratory volume Vetends to be temporarily overestimated and the inspiratory volume Vitends to be temporarily underestimated. In either case, the estimate oftidal volume V_(T) by the tidal volume estimation algorithm 4323 tendsto be temporarily higher than the actual tidal volume. The result insafety volume mode is that pressure support A is either inappropriatelyreduced, or increased more slowly than it should be. FIG. 7A contains agraph 7100 illustrating an example of such behaviour. The traces 7110 to7130 in the graph 7100 represent direct measurements of variousbreathing parameters, rather than those parameters as estimated by theRPT device 4000 using the algorithms 4300. The top trace 7110 representsthe treatment pressure Pt, oscillating with amplitude equal to thepressure support A. The middle trace 7120 represents the respiratoryflow rate Qr, oscillating between positive (inspiration) and negative(expiration) portions. The lower trace 7130 represents the integral ofrespiratory flow rate Qr, i.e. the instantaneous volume. The peak valueof the peaks in the trace 7130 therefore represents the tidal volumeV_(T) of each successive breath.

A leak suddenly appears at 7140. The pressure support fallssignificantly soon afterwards as the tidal volume is overestimated,before recovering to its previous value after a dozen or so breaths. Theresult is a temporary fall in the delivered tidal volume V_(T). The leakis resolved at 7150, and once again the pressure support fallssignificantly as the tidal volume is once again overestimated, takingmany breaths to recover its previous value, while the delivered tidalvolume V_(T) also falls significantly.

In one form of the present technology, the servo-control gain G inequation (2) may be adjusted dynamically so that this effect of a suddenchange in leak is reduced. As mentioned above, one effect of sudden leakappearance or resolution is to cause the estimates of inspiratory volumeVi and expiratory volume Ve to diverge temporarily. In oneimplementation, the servo-control gain G is therefore adjusted based ona difference between the estimates of inspiratory volume Vi andexpiratory volume Ve, so as to generally decrease as the differenceincreases. In one such implementation, the servo-control gain G isadjusted linearly between its default value and a lower value Gmin as arelative differential tidal volume dv varies between a low value dvminand a default value, e.g. 1. FIG. 8 illustrates one example of such anadjustment of the servo-control gain G, where the default value is one,the low value Gmin is 0.2, and the low value dvmin is also 0.2. Therelative differential tidal volume dv may be computed from the mostrecent breath as

$\begin{matrix}{{dv} = \frac{{{Vi} - {Ve}}}{V_{T}}} & (3)\end{matrix}$

with adjustments to ensure the denominator is never zero.

By this implementation, the rate of adjustment of pressure support A isreduced as a sudden leak appearance or resolution causes the estimatesof inspiratory volume Vi and expiratory volume Ve to divergetemporarily. Once these estimates converge as the leak flow rateestimation algorithm 4316 catches up with the sudden leak change, theservo-control gain G returns to its normal value of one. The effects ofthe sudden leak change on the servo-control of pressure support are thussmoothed out, giving a more stable therapy in the face of sudden changesin leak.

FIG. 7B contains a graph 7200 illustrating of the behaviour of thetherapy parameter determination algorithm 4329 in one suchimplementation of the present technology. As in FIG. 7A, the traces 7210to 7230 in the graph 7200 represent direct measurements of variousbreathing parameters, rather than those parameters as estimated by theRPT device 4000 using the algorithms 4300. The top trace 7210 representsthe treatment pressure Pt, the middle trace 7220 represents therespiratory flow rate Qr, and the lower trace 7230 represents theintegral of respiratory flow rate Qr, i.e. the instantaneous volume.

A leak suddenly appears at 7240, but the pressure support A falls farless than in the trace 7110 at the same stage. As a result, thedelivered tidal volume also falls by less than in the trace 7130 at thesame stage. The leak is resolved at 7250, and once again the pressuresupport A falls far less than in the trace 7110, resulting in a smallerfall in the delivered tidal volume than in the trace 7130 at the samestage. The delivered tidal volume V_(T) therefore varies significantlyless in the face of the sudden leak changes.

FIG. 9 shows a high level flow chart of a method 9000 of dynamicallyadjusting the servo-control gain G in the form of an algorithm that maybe implemented in, or as a separate module of, a therapy engine module4320 in a respiratory therapy apparatus such as RPT 4000. At block 9004,the algorithm is invoked upon detection of a sudden change in leak. Asshown in FIG. 7A, sudden leak detection may be made dependent on achange in pressure support or tidal volume. A sudden leak may bedetermined based on a rise in the relative differential tidal volume dv(see equation (3)) between the estimates of inspiratory volume Vi andexpiratory volume Ve above some threshold dvmin over some predeterminedtime period (e.g., a single or multiple breaths). Upon detection of thesudden leak, the servo-control gain G of the apparatus is adjusted atblock 9008 based on a difference between estimated volume values Vi andVe. As shown in FIG. 8 and as defined in equation (3), the adjustmentmay vary linearly between a default or normal upper value and a lowerminimum value, Gmin based on a relative differential tidal volume dv.The gain G is thus adjusted dynamically in response to sudden leak whichimproves the performance of apparatus incorporating the technology byservo-controlling the degree of support in response to the sudden leak.Thus, the effect of the leak is reduced and therefore the impact of theleak on the operation in safety volume mode is also reduced and may bereflected in the change of ventilatory support provided.

At decision diamond 9012, the difference between Vi and Ve is examinedto determine whether there has been convergence, e.g., the differencebetween Vi and Ve has converged within some predetermined limit such asrelative difference of 20% or less. If there is convergence, the gain Gis then returned to its normal or default value (e.g. 1) at block 9016.If there has been no convergence, then the gain G may be furtheradjusted at block 9008 depending on the relative difference between theestimated inspiratory and expiratory volumes Vi, Ve. This allows for anadditional level of dynamic adjustment. For example, until theconvergence criterion is met, the gain G may be adjusted linearly basedon the computed relative differential tidal volume dv as illustrated inFIG. 8.

In an alternative implementation, blocks 9004, 9012, and 9016 are notused, and block 9008 is repeatedly invoked to adjust the servo-controlgain G of the apparatus based on a difference between estimated volumevalues Vi and Ve.

As an example, a respiratory therapy apparatus that includes a pressuregenerator, transducer and controller may be configured to operate inaccordance with an aspect of the present technology. In this example,the pressure generator would generate a flow of air or gas useful inproviding ventilatory support to a patient. The transducer may they thengenerate a signal that represents one or more properties of the flow ofair. A controller would then process or analyse the signal to estimatevalues for inspiratory and expiratory volumes of one or more patientbreaths. The controller would also be configured to servo-control theventilatory support to adjust an estimated tidal volume toward a targetvolume using a gain that dynamically adjusts based on a differencebetween the estimated values for inspiratory and expiratory volumes.This results in improvement relating to the adjustment of the estimatedtidal volume in the presence of sudden leaks.

5.9 Glossary

For the purposes of the present technology disclosure, in certain formsof the present technology, one or more of the following definitions mayapply. In other forms of the present technology, alternative definitionsmay apply.

5.9.1 General

Air: In certain forms of the present technology, air may be taken tomean atmospheric air, and in other forms of the present technology airmay be taken to mean some other combination of breathable gases, e.g.atmospheric air enriched with oxygen.

Ambient: In certain forms of the present technology, the term ambientwill be taken to mean (i) external of the treatment system or patient,and (ii) immediately surrounding the treatment system or patient.

For example, ambient humidity with respect to a humidifier may be thehumidity of air immediately surrounding the humidifier, e.g. thehumidity in the room where a patient is sleeping. Such ambient humiditymay be different to the humidity outside the room where a patient issleeping.

In another example, ambient pressure may be the pressure immediatelysurrounding or external to the body.

In certain forms, ambient (e.g., acoustic) noise may be considered to bethe background noise level in the room where a patient is located, otherthan for example, noise generated by an RPT device or emanating from amask or patient interface. Ambient noise may be generated by sourcesoutside the room.

Automatic Positive Airway Pressure (APAP) therapy: CPAP therapy in whichthe treatment pressure is automatically adjustable, e.g. from breath tobreath, between minimum and maximum limits, depending on the presence orabsence of indications of SDB events.

Continuous Positive Airway Pressure (CPAP) therapy: Respiratory pressuretherapy in which the treatment pressure is approximately constantthrough a respiratory cycle of a patient. In some forms, the pressure atthe entrance to the airways will be slightly higher during exhalation,and slightly lower during inhalation. In some forms, the pressure willvary between different respiratory cycles of the patient, for example,being increased in response to detection of indications of partial upperairway obstruction, and decreased in the absence of indications ofpartial upper airway obstruction.

Flow rate: The volume (or mass) of air delivered per unit time. Flowrate may refer to an instantaneous quantity. In some cases, a referenceto flow rate will be a reference to a scalar quantity, namely a quantityhaving magnitude only. In other cases, a reference to flow rate will bea reference to a vector quantity, namely a quantity having bothmagnitude and direction. Flow rate may be given the symbol Q. ‘Flowrate’ is sometimes shortened to simply ‘flow’ or ‘airflow’.

In the example of patient respiration, a flow rate may be nominallypositive for the inspiratory portion of a breathing cycle of a patient,and hence negative for the expiratory portion of the breathing cycle ofa patient. Total flow rate, Qt, is the flow rate of air leaving the RPTdevice. Vent flow rate, Qv, is the flow rate of air leaving a vent toallow washout of exhaled gases. Leak flow rate, Ql, is the flow rate ofleak from a patient interface system or elsewhere. Respiratory flowrate, Qr, is the flow rate of air that is received into the patient'srespiratory system.

Humidifier: The word humidifier will be taken to mean a humidifyingapparatus constructed and arranged, or configured with a physicalstructure to be capable of providing a therapeutically beneficial amountof water (H₂O) vapour to a flow of air to ameliorate a medicalrespiratory condition of a patient.

Leak: The word leak will be taken to be a flow of air to or from ambientother than through the elements of the air circuit. In one example, leakmay occur as the result of an incomplete seal between a mask and apatient's face. In another example leak may occur in a swivel elbow tothe ambient. Leak may also encompass the air exhaled to ambient around atracheostomy tube in invasive ventilation.

Noise, conducted (acoustic): Conducted noise in the present documentrefers to noise which is carried to the patient by the pneumatic path,such as the air circuit and the patient interface as well as the airtherein. In one form, conducted noise may be quantified by measuringsound pressure levels at the end of an air circuit.

Noise, radiated (acoustic): Radiated noise in the present documentrefers to noise which is carried to the patient by the ambient air. Inone form, radiated noise may be quantified by measuring soundpower/pressure levels of the object in question according to ISO 3744.

Noise, vent (acoustic): Vent noise in the present document refers tonoise which is generated by the flow of air through any vents such asvent holes of the patient interface.

Patient: A person, whether or not they are suffering from a respiratorycondition.

Pressure: Force per unit area. Pressure may be expressed in a range ofunits, including cmH₂O, g-f/cm² and hectopascal. 1 cmH₂O is equal to 1g-f/cm² and is approximately 0.98 hectopascal. In this specification,unless otherwise stated, pressure is given in units of cmH₂O.

The pressure in the patient interface is given the symbol Pm, while thetreatment pressure, which represents a target value to be achieved bythe mask pressure Pm at the current instant of time, is given the symbolPt.

Respiratory Pressure Therapy (RPT): The application of a supply of airto an entrance to the airways at a treatment pressure that is typicallypositive with respect to atmosphere.

Ventilator: A mechanical device that provides pressure support to apatient to perform some or all of the work of breathing.

5.9.2 Respiratory Cycle

Apnea: According to some definitions, an apnea is said to have occurredwhen flow falls below a predetermined threshold for a duration, e.g. 10seconds. An obstructive apnea will be said to have occurred when,despite patient effort, some obstruction of the airway does not allowair to flow. A central apnea will be said to have occurred when an apneais detected that is due to a reduction in breathing effort, or theabsence of breathing effort, despite the airway being patent. A mixedapnea occurs when a reduction or absence of breathing effort coincideswith an obstructed airway.

Breathing rate: The rate of spontaneous respiration of a patient,usually measured in breaths per minute.

Duty cycle: The ratio of inhalation time, Ti to total breath time, Ttot.

Effort (breathing): The work done by a spontaneously breathing personattempting to breathe.

Expiratory portion of a breathing cycle: The period from the start ofexpiratory flow to the start of inspiratory flow.

Flow limitation: Flow limitation will be taken to be the state ofaffairs in a patient's respiration where an increase in effort by thepatient does not give rise to a corresponding increase in flow. Whereflow limitation occurs during an inspiratory portion of the breathingcycle it may be described as inspiratory flow limitation. Where flowlimitation occurs during an expiratory portion of the breathing cycle itmay be described as expiratory flow limitation.

Hypopnea: According to some definitions, a hypopnea is taken to be areduction in flow, but not a cessation of flow. In one form, a hypopneamay be said to have occurred when there is a reduction in flow below athreshold rate for a duration. A central hypopnea will be said to haveoccurred when a hypopnea is detected that is due to a reduction inbreathing effort. In one form in adults, either of the following may beregarded as being hypopneas:

-   -   (i) a 30% reduction in patient breathing for at least 10 seconds        plus an associated 4% desaturation; or    -   (ii) a reduction in patient breathing (but less than 50%) for at        least 10 seconds, with an associated desaturation of at least 3%        or an arousal.

Hyperpnea: An increase in flow to a level higher than normal.

Inspiratory portion of a breathing cycle: The period from the start ofinspiratory flow to the start of expiratory flow will be taken to be theinspiratory portion of a breathing cycle.

Patency (airway): The degree of the airway being open, or the extent towhich the airway is open. A patent airway is open. Airway patency may bequantified, for example with a value of one (1) being patent, and avalue of zero (0), being closed (obstructed).

Positive End-Expiratory Pressure (PEEP): The pressure above atmospherein the lungs that exists at the end of expiration.

Peak flow rate (Qpeak): The maximum value of flow rate during theinspiratory portion of the respiratory flow waveform.

Respiratory flow rate, patient airflow rate, respiratory airflow rate(Qr): These terms may be understood to refer to the RPT device'sestimate of respiratory flow rate, as opposed to “true respiratory flowrate” or “true respiratory flow rate”, which is the actual respiratoryflow rate experienced by the patient, usually expressed in litres perminute.

Tidal volume (V_(T)): The volume of air inhaled or exhaled during normalbreathing, when extra effort is not applied. In principle theinspiratory volume Vi (the volume of air inhaled) is equal to theexpiratory volume Ve (the volume of air exhaled), and therefore a singletidal volume V_(T) may be defined as equal to either quantity. Inpractice the tidal volume V_(T) is estimated as some combination, e.g.the mean, of these two quantities.

(inhalation) Time (Ti): The duration of the inspiratory portion of therespiratory flow rate waveform.

(exhalation) Time (Te): The duration of the expiratory portion of therespiratory flow rate waveform.

(total) Time (Ttot): The total duration between the start of oneinspiratory portion of a respiratory flow rate waveform and the start ofthe following inspiratory portion of the respiratory flow rate waveform.

Typical recent ventilation: The value of ventilation around which recentvalues of ventilation Vent over some predetermined timescale tend tocluster, that is, a measure of the central tendency of the recent valuesof ventilation.

Upper airway obstruction (UAO): includes both partial and total upperairway obstruction. This may be associated with a state of flowlimitation, in which the flow rate increases only slightly or may evendecrease as the pressure difference across the upper airway increases(Starling resistor behaviour).

Ventilation (Vent): A measure of a rate of gas being exchanged by thepatient's respiratory system. Measures of ventilation may include one orboth of inspiratory and expiratory flow, per unit time. When expressedas a volume per minute, this quantity is often referred to as “minuteventilation”. Minute ventilation is sometimes given simply as a volume,understood to be the volume per minute.

5.9.3 Ventilation

Adaptive Servo-Ventilator (ASV): A servo-ventilator that has achangeable, rather than fixed target property. The changeable targetproperty may be learned from some characteristic of the patient, forexample, a respiratory characteristic of the patient.

Backup rate: A parameter of a ventilator that establishes the minimumbreathing rate (typically in number of breaths per minute) that theventilator will deliver to the patient, if not triggered by spontaneousrespiratory effort.

Cycled: The termination of a ventilator's inspiratory phase. When aventilator delivers a breath to a spontaneously breathing patient, atthe end of the inspiratory portion of the breathing cycle, theventilator is said to be cycled to stop delivering the breath.

Expiratory positive airway pressure (EPAP): a base pressure, to which apressure varying within the breath is added to produce the desired maskpressure which the ventilator will attempt to achieve at a given time.

End expiratory pressure (EEP): Desired mask pressure which theventilator will attempt to achieve at the end of the expiratory portionof the breath. If the pressure waveform template Π(Φ) is zero-valued atthe end of expiration, i.e. Π(Φ)=0 when Φ=1, the EEP is equal to theEPAP.

Inspiratory positive airway pressure (IPAP): Maximum desired maskpressure which the ventilator will attempt to achieve during theinspiratory portion of the breath.

Pressure support: A number that is indicative of the increase inpressure during ventilator inspiration over that during ventilatorexpiration, and generally means the difference in pressure between themaximum value during inspiration and the base pressure (e.g.,PS=IPAP−EPAP). In some contexts pressure support means the differencewhich the ventilator aims to achieve, rather than what it actuallyachieves.

Servo-ventilator: A ventilator that measures or estimates some parameterof the patient's respiratory cycle and adjusts the level of pressuresupport to adjust the measured parameter towards a target value of theparameter.

Spontaneous/Timed (S/T): A mode of a ventilator or other device thatattempts to detect the initiation of a breath of a spontaneouslybreathing patient. If however, the device is unable to detect a breathwithin a predetermined period of time, the device will automaticallyinitiate delivery of the breath.

Swing: Equivalent term to pressure support.

Triggered: When a ventilator delivers a breath of air to a spontaneouslybreathing patient, it is said to be triggered to do so at the initiationof the respiratory portion of the breathing cycle by the patient'sefforts.

5.10 Other Remarks

A portion of the disclosure of this patent document contains materialwhich is subject to copyright protection. The copyright owner has noobjection to the facsimile reproduction by anyone of the patent documentor the patent disclosure, as it appears in Patent Office patent files orrecords, but otherwise reserves all copyright rights whatsoever.

Unless the context clearly dictates otherwise and where a range ofvalues is provided, it is understood that each intervening value, to thetenth of the unit of the lower limit, between the upper and lower limitof that range, and any other stated or intervening value in that statedrange is encompassed within the technology. The upper and lower limitsof these intervening ranges, which may be independently included in theintervening ranges, are also encompassed within the technology, subjectto any specifically excluded limit in the stated range. Where the statedrange includes one or both of the limits, ranges excluding either orboth of those included limits are also included in the technology.

Furthermore, where a value or values are stated herein as beingimplemented as part of the technology, it is understood that such valuesmay be approximated, unless otherwise stated, and such values may beutilized to any suitable significant digit to the extent that apractical technical implementation may permit or require it.

Unless defined otherwise, all technical and scientific terms used hereinhave the same meaning as commonly understood by one of ordinary skill inthe art to which this technology belongs. Although any methods andmaterials similar or equivalent to those described herein can also beused in the practice or testing of the present technology, a limitednumber of the exemplary methods and materials are described herein.

When a particular material is identified as being used to construct acomponent, obvious alternative materials with similar properties may beused as a substitute. Furthermore, unless specified to the contrary, anyand all components herein described are understood to be capable ofbeing manufactured and, as such, may be manufactured together orseparately.

It must be noted that as used herein and in the appended claims, thesingular forms “a”, “an”, and “the” include their plural equivalents,unless the context clearly dictates otherwise.

All publications mentioned herein are incorporated herein by referencein their entirety to disclose and describe the methods and/or materialswhich are the subject of those publications. The publications discussedherein are provided solely for their disclosure prior to the filing dateof the present application. Nothing herein is to be construed as anadmission that the present technology is not entitled to antedate suchpublication by virtue of prior invention. Further, the dates ofpublication provided may be different from the actual publication dates,which may need to be independently confirmed.

The terms “comprises” and “comprising” should be interpreted asreferring to elements, components, or steps in a non-exclusive manner,indicating that the referenced elements, components, or steps may bepresent, or utilized, or combined with other elements, components, orsteps that are not expressly referenced.

The subject headings used in the detailed description are included onlyfor the ease of reference of the reader and should not be used to limitthe subject matter found throughout the disclosure or the claims. Thesubject headings should not be used in construing the scope of theclaims or the claim limitations.

Although the technology herein has been described with reference toparticular examples, it is to be understood that these examples aremerely illustrative of the principles and applications of thetechnology. In some instances, the terminology and symbols may implyspecific details that are not required to practice the technology. Forexample, although the terms “first” and “second” may be used, unlessotherwise specified, they are not intended to indicate any order but maybe utilised to distinguish between distinct elements. Furthermore,although process steps in the methodologies may be described orillustrated in an order, such an ordering is not required. Those skilledin the art will recognize that such ordering may be modified and/oraspects thereof may be conducted concurrently or even synchronously.

It is therefore to be understood that numerous modifications may be madeto the illustrative examples and that other arrangements may be devisedwithout departing from the spirit and scope of the technology.

5.11 Reference Signs List

-   -   patient 1000    -   patient interface 3000    -   seal-forming structure 3100    -   plenum chamber 3200    -   structure 3300    -   vent 3400    -   connection port 3600    -   forehead support 3700    -   RPT device 4000    -   external housing 4010    -   upper portion 4012    -   portion 4014    -   panel s 4015    -   chassis 4016    -   handle 4018    -   pneumatic block 4020    -   air filters 4110    -   inlet air filter 4112    -   outlet air filter 4114    -   mufflers 4120    -   inlet muffler 4122    -   outlet muffler 4124    -   pressure generator 4140    -   blower 4142    -   motor 4144    -   anti-spill back valve 4160    -   air circuit 4170    -   electrical components 4200    -   PCBA 4202    -   power supply 4210    -   input devices 4220    -   central controller 4230    -   clock 4232    -   therapy device controller 4240    -   protection circuits 4250    -   memory 4260    -   transducers 4270    -   pressure sensor 4272    -   flow rate sensors 4274    -   motor speed transducer 4276    -   data communication interface 4280    -   remote external communication network 4282    -   local external communication network 4284    -   remote external device 4286    -   local external device 4288    -   output device 4290    -   display driver 4292    -   display 4294    -   algorithms 4300    -   pre-processing module 4310    -   pressure compensation algorithm 4312    -   vent flow rate estimation algorithm 4314    -   leak flow rate estimation algorithm 4316    -   respiratory flow rate estimation algorithm 4318    -   therapy engine module 4320    -   phase determination algorithm 4321    -   waveform determination algorithm 4322    -   tidal volume estimation algorithm 4323    -   therapy parameter determination algorithm 4329    -   therapy control module 4330    -   fault condition detection methods 4340    -   humidifier 5000    -   humidifier inlet 5002    -   humidifier outlet 5004    -   humidifier base 5006    -   humidifier reservoir 5110    -   humidifier reservoir dock 5130    -   heating element 5240    -   graph 7100    -   trace 7110    -   trace 7120    -   trace 7130    -   graph 7200    -   trace 7210    -   trace 7220    -   trace 7230    -   sudden leak change 7240    -   leak resolution 7250    -   method 9000    -   block 9004    -   block 9008    -   decision diamond 9012    -   block 9016

1. Apparatus for treating a respiratory disorder in a patient, theapparatus comprising: a pressure generator configured to generate a flowof air so as to provide ventilatory support to the patient; a transducerconfigured to generate a signal representing a property of the flow ofair; and a controller configured to: analyse the signal to estimate aninspiratory volume and an expiratory volume of a breath of the patient;and servo-control a degree of the ventilatory support to adjust anestimated tidal volume toward a target tidal volume, wherein a gain ofthe servo-control is dependent on a difference between the estimatedinspiratory volume and the estimated expiratory volume.
 2. The apparatusof claim 1, wherein the gain decreases as the difference between theestimated inspiratory volume and the estimated expiratory volumeincreases.
 3. The apparatus of any one of claims 1 to 2, wherein thegain is dependent on the difference between the estimated inspiratoryvolume and the estimated expiratory volume relative to the estimatedtidal volume.
 4. The apparatus of any one of claims 1 to 3, wherein thegain is dependent on the absolute magnitude of the difference betweenthe estimated inspiratory volume and the estimated expiratory volume. 5.The apparatus of any one of claims 1 to 4, wherein the degree of theventilatory support is a pressure support of the ventilatory support. 6.A method of operating a respiratory treatment apparatus configured togenerate a flow of air so as to provide ventilatory support to apatient, the method comprising: measuring a property of the flow of air,using a transducer; analysing, in a controller, the measured property toestimate an inspiratory volume and an expiratory volume of a breath ofthe patient; calculating, in a controller, a gain dependent on adifference between the estimated inspiratory volume and the estimatedexpiratory volume; and servo-controlling, by a controller, a degree ofthe ventilatory support using the calculated gain to adjust an estimatedtidal volume for the patient toward a target tidal volume.
 7. The methodof claim 6, wherein the calculating decreases the gain as the differencebetween the estimated inspiratory volume and the estimated expiratoryvolume increases.
 8. The method of any one of claims 6 to 7, wherein thegain is dependent on the difference between the estimated inspiratoryvolume and the estimated expiratory volume relative to the estimatedtidal volume.
 9. The method of any one of claims 6 to 8, wherein thegain is dependent on the absolute magnitude of the difference betweenthe estimated inspiratory volume and the estimated expiratory volume.10. The method of any one of claims 6 to 9, wherein the degree of theventilatory support is a pressure support of the ventilatory support.11. A system for treating a respiratory disorder in a patient,comprising: means for generating a flow of air so as to provideventilatory support to the patient; means for generating a signalrepresenting a property of the flow of air; means for analysing thesignal to estimate an inspiratory volume and an expiratory volume of abreath of the patient; and means for servo-controlling a degree of theventilatory support to adjust an estimated tidal volume toward a targettidal volume, wherein a gain of the servo-control is dependent on adifference between the estimated inspiratory volume and the estimatedexpiratory volume.
 12. Apparatus for treating a respiratory disorder ina patient, the apparatus comprising: a blower configured to deliver asupply of air that provides ventilatory support to the patient; atransducer configured to generate a signal representing a property ofthe supply of air; and a controller configured to: analyse the signal toestimate an inspiratory volume and an expiratory volume of a breath ofthe patient; and adjust servo-control gain based on a difference betweenthe estimated inspiratory volume and the estimated expiratory volume.13. The apparatus of claim 12, wherein the controller adjusts theservo-control gain so that the gain decreases as the difference betweenthe estimated inspiratory volume and the estimated expiratory volumeincreases.
 14. The apparatus of any one of claims 12 to 13, wherein anadjustment in servo-control gain causes a reduction in a rate ofadjustment of pressure support of the supply of air.
 15. The apparatusof any one of claims 12 to 14, wherein the controller is configured toadjust the servo-control gain when the difference is greater than orequal to 20%.
 16. The apparatus of any one of claims 12 to 15, whereinthe controller is further configured to adjust the servo-control gain toa default value when the difference returns to a value that is less than20%.